Cyclic-step quenching of micro-alloyed steel for enhancing mechanical properties: a novel heat treatment process implementation

Abstract

Abstract This work carried out a novel quench procedure known as cyclic-step quenching. A low-alloy low-carbon steel was prepared into five categories of samples designated as As-Received (AR), Cyclic-Step Quenched steel (CSQ_1, CSQ_2 and CSQ_3) and Straight Quenched steel (SQ). CSQ samples were heated in muffle furnace to austenization temperature 900 °C and held for 10 min. Samples were then cooled in the furnace to intercritical temperature 750 °C before quenching in vegetable oil. The process was repeated once and twice for samples CSQ_2 and CSQ_3respectively. Sample SQ was austenized as done for CSQ but subsequently quenched in vegetable oil without being cooled to intercritical temperature. All samples were subjected to microstructural, tensile, hardness and impact toughness tests. Results obtained revealed that Ultimate Tensile Strength (UTS), and product of UTS & percentage elongation improved from 692.09 to 919.39 MPa and 11.63 to 31.63 GPa-% respectively, at the second cycle of step quench due to stability and refinement of austenite phase. Brinell hardness value obtained showed that second cycle of step quench recorded highest value of 194.98 BHN. All quenched samples recorded lowered impact toughness values than the as-received. The work reported here combined the refining effect of cyclic quenching and ferrite phase precipitation from step quenching to improve formability of experimented steel.